The present invention relates to expandable ceramic fiber felt and a manufacturing process for making the same.
Ceramic refractory fibrous materials of the alumina-silica, alumina and zirconia type have been used for thermal insulation for many years. The usual fiber is commercially manufactured by a jet of air or steam shattering a stream of molten ceramic with the so formed fibers collected in bulk form. Ordinarily, the collector bulk fiber is compressed to form fiber blankets or sheets of a preferred bulk density. Felt pads of ceramic fibers can be formed and used at high-temperature heat insulating material as well. These felt pads are inserted into the gaps of expansion joints or seals of furnace walls.
Conventional expandable ceramic fiber felt with vermiculite incorporated therein has been found to be an ineffective insulator because the vermiculite withstand only up to 1100.degree. C., whereas the ceramic fibers alone withstand a temperature greater than 1260.degree. C. Furthermore the vermiculite reacts with the ceramic fibers at elevated temperature decreasing the heat resistance of the fibers.
It is the object of this invention to provide an expandable ceramic fiber felt which expands upon heating.
According to this invention, this object is achieved by an expandable ceramic fiber felt which is comprised of ceramic fiber, expandable graphite flakes, and an organic binder.
The graphite flakes which expand upon heating are produced by treating natural graphite flakes with an oxidizing agent such as a mixture of sulfuric acid and nitric acid, followed by a water rinse. The natural graphite flakes then form openings between the networks of carbon atom layers and pass to expandable graphite flakes which expand in the laminated direction when heated to 350.degree. to 600.degree. C. The flakes are uniformly dispersed in the ceramic fiber felt in such a manner that the greater surface area of the graphite flake is parallel to the orientated direction of the ceramic fibers. The graphite flakes are present in amounts of about 2 to 20 weight percent based on the total weight of the ceramic fibers and graphite flakes.